In the forthcoming evolution of the mobile cellular standards like GSM and WCDMA, new transmission techniques like OFDM are likely to occur. Furthermore, in order to have a smooth migration from the existing cellular systems to the new high capacity high data rate system in existing radio spectrum, the new system has to be able to operate in a flexible bandwidth. An example of such a new flexible cellular system is 3G Long Term Evolution (3G LTE) that can be seen as an evolution of the 3G WCDMA standard. This system will use OFDM as the downlink transmission scheme and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be supported on the largest bandwidth. LTE will support both FDD and TDD as uplink/downlink duplexing schemes. Furthermore, LTE will also support multicast/broadcast services (MBSFN) on the same carrier as unicast data.
An essential part in any cellular system is support of mobility, i.e., the possibility to move the connection between the terminal and the network from one cell to another cell. To support this, neighboring cell measurements are used. While the connection is maintained in a serving cell, the terminal measures on some well defined signal in neighboring cells and reports the measurement result to the network. The network can then make a decision, for example based on a signal-to-noise ratio measurement made by the terminal, whether the connection should be moved from the serving cell to a new cell.
In order to carry out downlink coherent demodulation, the mobile terminal needs estimates of the downlink channel. A straightforward way to enable channel estimation in case of OFDM transmission is to insert known reference symbols into the OFDM time-frequency grid. In LTE, these reference symbols are jointly referred to as the LTE downlink reference signals.
FIG. 1 is grid in the time frequency domain, with each square in the grid representing one subcarrier of one OFDM symbol. It serves to demonstrate the LTE downlink reference-signal structure assuming normal cyclic prefix, i.e. seven OFDM symbols per slot. As illustrated in FIG. 1, downlink reference symbols are inserted within the first and the third last OFDM symbol of each slot and with a frequency-domain spacing of six subcarriers. Furthermore, there is a frequency-domain staggering of three subcarriers between the first and second reference symbols. Within each resource block, consisting of twelve subcarriers during one slot, there are thus four reference symbols. This is true for all sub-frames except sub-frames used for MBSFN-based broadcast/multicast, see further below.
The structure in FIG. 1 illustrates the reference-signal structure for the case of a single antenna. For various multi-antenna transmission techniques, there is typically one reference signal transmitted for each antenna (the term ‘antenna port’ is used in the 3GPP specifications) and the location of the reference signals for the different antennas may be different.
The reference signals can also be used for other purposes than coherent demodulation. One such example is neighboring cell measurements for mobility, where the terminal measures on the reference signal in neighboring cells to support mobility as described above.
One important part of the LTE requirements in terms of spectrum flexibility is the possibility to deploy LTE-based radio-access in both paired and unpaired spectrum, i.e., LTE should support both FDD- and TDD-based duplex arrangements. Frequency Division Duplex (FDD) as illustrated in the left part of FIG. 2, implies that downlink and uplink transmission take place in different, sufficiently separated, frequency bands. Time Division Duplex (TDD), as illustrated in the right part of FIG. 2, implies that downlink and uplink transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired spectrum, whereas FDD requires paired spectrum.
To support TDD operation, a guard time between downlink and uplink timeslots is needed. This can be created by omitting one or several OFDM symbols (“puncturing”) in the last sub-frame before the downlink-to-uplink switch. In case a long guard time is needed, some of the reference symbols may need to be punctured in the last sub-frame prior to the switchpoint. The non-punctured part of a subframe used for downlink transmission is sometimes referred to as DwPTS.
In case of TDD operation, uplink and downlink transmission activity should be coordinated between neighboring cells. If this is not done, uplink transmission in one cell may interfere with downlink transmission in the neighboring cell (and vice versa) as illustrated in FIG. 3. Related to measurements, the terminal should only make neighbouring cell measurements during downlink transmission slots.
Multi-cell broadcast implies transmission of the same information from multiple cells. By exploiting this at the terminal, effectively using signal power from multiple cell sites at the detection, a substantial improvement in coverage, or in higher broadcast data rates, can be achieved. In LTE, this is implemented by transmitting not only identical signals from multiple cell sites, with identical coding and modulation, but also synchronize the transmission timing between the cells, the signal at the mobile terminal will appear exactly as a signal transmitted from a single cell site and subject to multi-path propagation. Due to the OFDM robustness to multi-path propagation, such multi-cell transmission, also referred to as Multicast-Broadcast Single Frequency Network (MBSFN) transmission, will then not only improve the received signal strength but also eliminate the inter-cell interference. Thus, with OFDM, multi-cell broadcast/multicast capacity may eventually only be limited by noise and can then, in case of small cells, reach extremely high values.
It should also be noted that the use of MBSFN transmission for multi-cell broadcast/multicast assumes the use of tight synchronization and time alignment of the signals transmitted from different cell sites.
For MBSFN, a different reference signal structure is used as illustrated in FIG. 4. This is needed as the effective channel seen by the terminal in case of MBSFN transmission appears as more frequency-selective than a single-cell unicast transmission. Thus, as unicast data and MBSFN transmissions are time multiplexed in different time slots, the reference signal structure will differ between slots in case of a mixed carrier transmitting both unicast and MBSFN services. In MBSFN sub-frames, only part of the cell-specific reference signal is present, it occurs in some the first OFDM symbols of the sub-frame as disclosed in FIG. 4. The OFDM symbols carrying cell-specific reference symbol in the MBSFN sub-frame is a sub-set of the symbols used in a normal sub-frame for carrying cell-specific reference symbols, as can be concluded by comparing FIG. 1 and FIG. 4.
Typically, the terminal assumes the same configuration in the neighboring cell as the current cell. In case neighboring cells are configured differently, e.g., different guard times are used in neighboring cells or the MBSFN sub-frame are allocated differently in neighboring cells, the measurements made in the terminal would not correctly reflect the situation.